1. Field of the Invention
The present invention relates to a variable capacitor whose capacitance is changed in accordance with a pressure or a displacement applied thereto from the outside, and a position indicator using the same.
2. Description of the Related Art
For example, as shown in Japanese Patent Laid-Open No. Sho 64-53223 (hereinafter referred to as Patent Document 1), there is known a position input device composed of a pen type position indicator and a position detector. The position detector includes a sensor portion for detecting a position indicated by the position indicator.
In general, the sensor portion has a flat plate-like shape, and includes an indication detecting plane for detecting an input position indicated by the position indicator. Also, in the case of an electromagnetic induction type position detector, the sensor portion is provided with a large number of long and thin loop coils which are arranged in an X-axis direction and in a Y-axis direction. On the other hand, the position indicator is provided with a resonance circuit composed of a coil and a capacitor.
The position detector causes currents (transmission currents for excitation) having respective specific frequencies to flow through the loop coils of the sensor portion, and causes the loop coils of the sensor portion to generate magnetic fields from the loop coils. Then, when the position indicator is close to the loop coil which is generating the magnetic field, the resonance circuit of the position indicator resonates with the loop coil based on electromagnetic induction to generate an induced magnetic field. Next, the generation of the magnetic field from the loop coil is stopped. And, next, the induced magnetic field generated from the resonance circuit of the position indicator is received by the loop coil, thereby detecting a signal current (received current) being caused to flow through that loop coil. The position detector carries out this operation on every loop coil on a one-by-one basis, thereby detecting the position of the position indicator based on the received current.
The existing position indicator includes a core (rod) body for detecting a pen pressure, and also includes a mechanism for making an inductance of the coil variable in accordance with the pen pressure applied to the core body. As a result, in the position indicator, a resonance frequency of the resonance circuit is changed in accordance with a change in pen pressure. Thus, the position detector detects the change (phase change) in resonance frequency, thereby additionally detecting the pen pressure applied to the position indicator. The existing position indicator is constructed in such a manner.
However, the position indicator described in Patent Document 1 involves such a problem that, due to its construction, a certain amount of axial stroke of the core body is generated and also a change width of the resonance frequency cannot be made sufficiently large. For the purpose of solving this problem, the applicant of this application has proposed a position indicator using a variable capacitor whose capacitance is changed in accordance with a pen pressure applied to a core body in Japanese Patent Laid-Open Nos. Hei 04-96212 and Hei 05-275283 (hereinafter referred to as Patent Documents 2 and 3, respectively), and Japanese Patent Application No. 2008-305556 (hereinafter referred to as “the previous application,” published as Japanese Patent Laid-Open No. 2010-129920). In short, such position indicator has a configuration, in which a variable capacitor is provided in a resonance circuit of the position indicator, and a capacitance of the variable capacitor is changed in accordance with a pen pressure, thereby changing a resonance frequency of the resonance circuit.
Specifically, the position indicator described in Patent Document 2 uses a variable capacitor in which a first electrode is provided on one surface of a dielectric, and a second electrode is provided on the other surface of the dielectric through a spacer interposed between the dielectric and the second electrode. The second electrode can be pressed against the dielectric through an elastic body by a core (rod) body. In this case, the variable capacitor is constructed in such a way that the second electrode comes to contact the dielectric through the elastic body in accordance with the pressing force (the pen pressure) applied to the core body, and the capacitance of the variable capacitor is changed in correspondence with the contacting state. As a result, it is possible to solve the problem associated with the position indicator, described in Patent Document 1, in which the resonance frequency is changed in accordance with the change in inductance of the coil.
The position indicator described in Patent Document 3 uses a variable capacitor in which a first electrode is provided on one surface of a dielectric, and a second electrode made of a conductive rubber having both flexibility and elasticity is disposed on the other surface of the dielectric through a spacer interposed between the dielectric and second electrode. The variable capacitor is constructed in such a way that the second electrode made of the conductive rubber comes to contact the dielectric in accordance with the pressing force (the pen pressure) applied to the core body, and the capacitance of the variable capacitor is changed in accordance with the contacting state. In this case, since the second electrode is made of the conductive rubber having both the flexibility and the elasticity, it is possible to realize the position indicator from which a stable output value is obtained with a smaller pen pressure as compared with the position indicator described in Patent Document 2.
The invention described in the previous application, that is, Japanese Patent Application JP 2008-305556, uses a variable capacitor having a dielectric and first and second electrodes, similarly to each of the cases of Patent Document 1 and 2. However, the variable capacitor is constructed in such a way that a positional relationship between the dielectric and the second electrode can be reliably returned back to an initial state, thereby enhancing the durability. Specifically, a dielectric, a terminal member for biasing the dielectric, and a conductive member composing the second electrode are provided in a holder, wherein the conductive member and the terminal member are provided so as to sandwich the dielectric therebetween. Also, an elastic member is provided in a circumference of the conductive member for pressing the conductive member away from the dielectric. The conductive member is pressed in the direction of the dielectric when the pressing force is applied to the core body.
As described above, the applicant of this application has made various inventions directed to a position indicator used together with a tablet in consideration of various factors, such as its use or application and the durability. It is to be noted that the inventions of Patent Documents 1 to 3 and the previous application have their respective unique features and, thus, are applied in the position indicators which are widely supported by end users.